The invention concerns a process for regenerating aqueous urea solutions used in dewaxing mineral oil fractions by reacting said aqueous urea solutions with said mineral oil fractions, thereby forming solid n-paraffin/urea adducts; recovering said aqueous urea solutions by means of decomposition of the adducts and, optionally, purifying and, optionally, reconcentrating, and reusing said aqueous urea solutions for the formation of adducts.
The process, whereby n-paraffins contained in hydrocarbon mixtures or mineral oil fractions are separated from the same, is known. In this process, the mineral oils, preferably diluted with an organic solvent, are reacted with urea or concentrated urea solutions; the solid inclusion compounds or adducts thereby formed, comprising n-paraffins and urea, are separated from the dewaxed mineral oil, purified and finally decomposed into urea or urea solution and the crude n-paraffins. This basic form of the so-called urea dewaxing process is performed, for the most part, continuously and on a large commercial scale, in a number of different embodiments.
In one of said embodiments, disclosed for example in German Pat. No. 1,000,951, the mineral oil to be dewaxed is treated with an aqueous urea solution containing about 10-40 weight % water, and the adducts so obtained, after being separated from the dewaxed mineral oil, are purified by washing with organic solvent and finally heated, suitably by the introduction of steam, together with the addition of water. This decomposition of the adduct yields an aqueous urea solution with reduced urea concentration, which solution must be passed to a thickener or concentrator and there be restored to its original water content of about 10-40 weight %, so that it can be re-used for the formation of adducts. A similar process for reconcentrating a urea solution for recycling into the circulatory system is disclosed in German Pat. No. 1,098,657.
However, the adduct formation capacity of aqueous solutions, recycled in this manner in the system, gradually diminishes despite reconcentration, cf., for example, "Erdol und Kohle", Vol. 11 (1958), No. 9, p. 619. According to the process disclosed in German Pat. No. 1,003,894, a portion or part of the stream, about 10-20%, should be drawn continuously from said aqueous urea solutions constantly being recycled in the system. This partial stream should be passed through a regeneration zone filled with activated carbon or a similar adsorbent and then reunited with the main stream and passed to the thickener for reconcentration.
It has since been shown that in certain cases the purification process disclosed in German Pat. No. 1,003,894 is inadequate or does not remain effective long enough, with the result that the time required to form separable adducts is too great. Therefore the regenerated urea solution must eventually be disposed of although it still contains the optimum urea concentration necessary for adduct formation. Apparently, despite regenerative treatment, substances gradually build up in the circulating urea solution which inhibit adduct formation, and which either are derived from the charge mineral oil or are formed by hydrolytic decomposition of the urea.
Purification of the diluted urea solutions prior to reconcentration by means of extraction with solvents was disclosed in German Offenlegungsschrift No. 1,919,663. This known purification process is concerned, however, solely with the separation of oily impurities and cannot appreciably prolong the effective life of the aqueous urea solutions purified by this means.
It is the object of this invention to develop a simple purification process for aqueous urea solutions which ensures uninterrupted re-use of the purified urea solutions for dewaxing of mineral oils over a considerably longer period of time than was possible in previous purification processes.